The present invention relates to a document exposure apparatus used in an image forming apparatus such as a copying machine or in an image reading apparatus such as a facsimile device. More particularly, the invention relates to a document exposing apparatus which can protect the eyes of an operator against illuminating light leaking from the machine.
In a copying machine, for example, a document original is located on a platen glass, and is illuminated with light emitted from an exposure lamp located under the platen glass. The light reflected by the image surface of the original is imaged on the surface of a charged photoreceptor so that a latent electrostatic image is formed on the surface of the photoreceptor. In some image forming machines, the reflected light is imaged on an image sensor where it is converted into electrical signals. In any case, to read the original image at a high contrast, light of satisfactorily high intensity must be used for illuminating the original.
When a normal original is copied, the rear side of the original located on the platen glass is covered with a document press cover. Accordingly, there is no danger that the illuminating light directly hits the eyes of an operator. When a thick document, for example, a book, is copied, it is frequently copied leaving the press cover open. In this case, there is a possibility that the illuminating light leaking around the document hits the eyes of an operator, possibly damaging the eyes. When an operator mistakenly pushes a start button without a document on the platen and the press cover is left open, more intensive light will enter the operator's eyes.
There have been many proposals to avoid the dangers of the leaked illuminating light hitting the eyes. Japanese Utility Model Publication No. Sho. 53-46016 discloses an exposure safety device in which the copying operation is permitted only when the press cover is substantially closed. Japanese Utility Model Publication No. Sho. 58-12176 discloses a copying machine in which a light-shielding sheet, disposed under the platen glass in a withdrawal manner, is manually withdrawn to cover the area around the region where the document original is placed to shut off the leaking light. Japanese Patent Unexamined Publication No. Sho. 59-52230 discloses a document operating device in which a liquid crystal board as a platen is controlled so that only the exposure area of the platen becomes transparent.
In the exposure safety device disclosed in Japanese Utility Model Publication No. Sho. 53-46016, the copying operation is permitted only when the press cover is substantially closed. To copy a book, for example, the press cover must be opened and closed every time the page is turned over. This manual operation is troublesome. Further, when the object to be copied is thick, such as a thick book, the press cover cannot be closed, so that the machine will not operate for copying.
In the copying machine disclosed in Japanese Utility Model Publication No. Sho. 58-12176, the amount of withdrawing of the light-shielding sheet must be varied in accordance with the size of an original. Thus, the operation is troublesome. Further, the light-shielding sheet and a drive mechanism for moving the sheet for withdrawal are required for the copying machine. This leads to increase of cost to manufacture.
In the device disclosed in Japanese Patent Unexamined Publication No. Sho. 59-52230, a special component such as the liquid crystal board in which the transparency is electrically controlled, is required for shutting off the leaking light. Use of the special component leads to increase of cost. Further, the control circuit for controlling the liquid crystal board is complicated since the transparent region must be moved in synchronism with movement of a light source.
In a case where the illuminating light has such a high intensity that for a short time exposure of the eyes to the illuminating light, the eyes suffer from decline of luminosity factor for blue, retina burn, cataract by infrared-rays, cornea burn, etc., the leaking light must be completely shut off by using the conventional structure as described above. Actually, normal exposure devices rarely use the illuminating light of such a high intensity. However, it is necessary to keep a quantity of accumulated light exposure within a predetermined value, in order to protect the retina from photochemical damage of the eyes which would be caused by a long time exposure of the eyes to illuminating light.